The present invention relates to a method for fabricating a photomask, and more particularly, to a method for fabricating a photomask including a phase shift layer.
Various patterns in semiconductor devices are formed by photolithography. Recently, as semiconductor devices become highly integrated, various technologies for forming ultra-fine patterns are developing. In order to improve the resolution by using a mask pattern, patterns are formed using a phase shift mask including a phase shifter.
The phase shift mask increases resolution or depth of focus by using interference or partial interference of light to expose patterns having a desired size. That is, when light passes through a mask substrate, a phase difference of the light occurs according to whether the phase shifter exists or not. As a result, the light passing through the phase shifter has a reverse phase with respect to light passing through the other portion of the phase shift mask. Accordingly, since the light passing through a light transmitting portion and the phase shifter are out of phase, light strength becomes zero in a boundary area of the pattern to increase the resolution by disposing the shifter at an edge of the mask pattern.
In contrast to conventional fine pattern formation methods, a method using the phase shift mask can improve a resolution limit of the mask by approximately 30% by only changing the mask without adding a new apparatus. Therefore, the method is regarded as a next-generation semiconductor manufacturing technique. Examples of the phase shift mask include a half tone phase shift mask (half tone PSM) in which the transmission coefficient increases from zero to a predetermined level or value above zero. In the half tone PSM, a molybdenum silicon nitride (MoSiN)-based half tone phase shift layer is disposed between a chromium (Cr) light blocking layer of a conventional binary mask and a quartz (Qz) substrate.
However, nuclei of particles are adhered to the half tone phase shift layer and the quartz (Qz) substrate due to a catalytic reaction caused by exposure of light and the storage environment during the formation of a wafer, and then moisture is held to the nuclei of the particles to increase the size of the particles. Therefore, the particles are formed around fine patterns formed on the mask or on the quartz (Qz) substrate to thereby cause defective patterns. In some cases, it is so serious that the photomask must fall into disuse.